I make quesadillas quite often actually. When you make them yourself it is SO much easier to keep track of the calories.
I make steak and cheese quesadilla's, sometimes just plain cheese. That's my favorite quick supper fix
I have found an offbrand, frozen, Philly Cheesesteak style, 4-pack of steaks at Wal-Mart. Anyway, they are 280 calories per steak. I use half of the steak per quesadilla.
I sprinkle a type of fiesta blend cheese (80 cals per serving) lightly on a tortilla(100 calories) and set it to the side.
I cook the steak in the pan, then drain most of the grease. For seasoning, if you want it to taste Tex-Mex add a dash of chili pepper to the steak while you cook it. I usually season mine with Tony's and salt and pepper. If you want to sautee any onions or bell peppers, do so now in the remaining steak grease lol.
I put half of the steak in the tortilla on top of the cheese, fold it over. Then I grill it in the pan like a grilled cheese sandwich. Or if you don't want that extra grease just pop it in the microwave till the cheese melts. (Just make sure you sprinkled a little water on the tortilla before hand or it will dry out in the microwave)
So that's about 320 calories for a pretty delicious and filling quesadilla. If you are a Salsa person, you could put some Salsa in it for about 35 extra cals.
I don't like sauces on mine, but I make one for my BF out of Salsa, Mayo, Mustard, Ketchup, and White Queso dip (Sounds nasty but it's actually good, just to fatty for my tastes)